Solvent-based coating compositions generally include a binder polymer and a crosslinking agent, which cross link upon application to produce coatings having excellent coating properties. One problem associated with such coating compositions is the relatively longer time required to cure these compositions. Such longer cure times cut down the productivity in automotive refinish shops by requiring the automobile or truck to remain for a longer period of time in the area in which it was spray coated. A rapid initial cure is thus desirable to produce a coating that can be readily sanded or buffed without fouling sandpaper. Such a rapid initial cure permits the user to readily remove coated automobile or trucks bodies out of the spray booths and allow them to fully cure at other convenient locations. As a result, productivity of coating autobodies can be improved substantially.
Attempts have been made to decrease the curing time of these two pack-coating compositions by using more reactive components or catalysts. However, while the use of such components decreases curing time, the higher reactivity of such components usually reduces the time to gelation or pot life of the coating composition as well. In the most extreme case, instant gelation of the composition can occur when, for example, the polyisocyanate is mixed with a polymer having reactive amine groups. Thus, a need still exists for a two-pack coating composition that cures rapidly while still having an acceptable pot life.
Attempts have been made to provide for a coating composition that cures rapidly after application. A commonly assigned U.S. Pat. No. 5,859,136 discloses a coating composition containing a dispersed core-shell acrylic polymer having a core polymerized from ethylenically unsaturated monomers containing amine functional groups. These amine groups positioned in the core are not readily available to react with the crosslinking agent, such as an isocyanate, which has to penetrate the shell to reach the core to crosslink with amine functionality. Applicants have come up with a novel alternate approach to reduce the cure time of the coating composition while still providing acceptable pot life and coating properties.